(1) Field of the Invention
The invention relates to an electrophoretic display device comprising at least a pixel with an electrophoretic medium in a main reservoir comprising at least one electrode, and means via which the pixel can be brought to different optical states. Where an exemplary switching electrode is mentioned in this application, it may be divided, if desired, into a plurality of sub-electrodes which are fed with one and the same voltage externally or via switching elements.
(2) Description of the Related Art
Electrophoretic display devices are based on the movement of charged, usually colored particles under the influence of an electric field between two extreme states having a different light transmissivity or light reflection. With these devices, dark (colored) characters can be imaged on a light (colored) background, and vice versa.
Electrophoretic display devices are therefore notably used in display devices taking over the function of paper, referred to as the “paper white” applications (electronic newspapers, electronic diaries).
In the known electrophoretic display devices with an electrophoretic medium between two switching electrodes, the switching electrodes are provided with drive voltages. In this case, the pixel can be brought to only two extreme optical states. One of the switching electrodes is then realized, for example, as two mutually interconnected narrow conducting strips on the upper side of a display element. In the case of a positive voltage across this switching electrode with respect to a bottom electrode, which covers the entire bottom surface of the display element, particles (which are negatively charged in this example) move towards the potential plane which is defined by the two interconnected narrow conducting strips. The (negatively) charged particles spread across the front surface of the display element (pixel) which then assumes the color of the charged particles. In the case of a negative voltage across the switching electrode with respect to the bottom electrode, the (negatively) charged particles spread on the bottom surface so that the display element (pixel) assumes the color of the liquid.
In practice there is an ever increasing need of displaying intermediate optical states (referred to as grey values). Known methods of introducing grey values are usually inadequate. For example, electrophoretic display devices are too slow to introduce grey values via time-weighted drive periods (time ratio grey scale). Division of the pixel into different surfaces (area ratio grey scale) usually requires barriers between the different sub-pixels so as to prevent mutual crosstalk.
Moreover, the number of electrodes required for the drive strongly increases in multicolor display devices.